Lithium batteries are commercially available in a variety of sizes employing many different electrochemistries. The increasing demand for such batteries results in great part from the high voltage of individual cells and the high energy density that generally characterizes these types of batteries. Small primary batteries employing lithium metal anodes are often employed in applications involving consumer electronics. Large lithium metal anode batteries are used in remote or military applications but are considered too dangerous for consumer usage. Rechargeable lithium batteries can offer many advantages over competing rechargeable non-lithium batteries or primary (non-rechargeable) lithium batteries. Interest in lithium ion type batteries in particular, wherein two different lithium insertion compounds are used as cathode and anode, has increased dramatically. The first commercial product based on a lithium ion type electrochemistry has recently been made available by Sony Energy Tec Inc.
Lithium ion electrochemistries under development generally employ a partially graphitized carbon or graphite as the anode, an air stable lithium transition metal oxide as the cathode, and a suitable non-aqueous electrolyte. Batteries made with such electrochemistries generally contain no metallic lithium on assembly. The lithium to be cycled as ions during operation of the battery is normally incorporated into one of the electrodes. Lithiated carbons or graphites are not stable in air however. The inserted lithium therein has a small binding energy and is extracted and reacts when exposed to air. Thus, it is difficult to use lithiated carbons or graphites in a manufacturing process for lithium ion batteries. Instead, all the lithium is normally incorporated into the cathode since many suitable lithium transition metal oxide materials can be prepared and are stable in air. An additional requirement of a lithium transition metal oxide to be used as a cathode is that the lithium ions are mobile and thus can be quickly extracted electrochemically.
Examples of suitable cathode materials for lithium ion batteries include LiNiO.sub.2, LiCoO.sub.2 and LiMn.sub.2 O.sub.4. The theoretical capacities for these materials are 275, 274, and 148 mAh/gram respectively, if all the lithium present could be used. However, for LiNiO.sub.2 and LiCoO.sub.2 only about 1/2 of the theoretical capacity can be used in a reversible manner. Further reversible lithiation of some of these materials is possible, thereby extending the capacity available per mole of cathode material. LiNiO.sub.2, for example, can be further lithiated to Li.sub.2 NiO.sub.2, but the latter compound is not stable in air. LiMn.sub.2 O.sub.4 can be further lithiated to Li.sub.2 Mn.sub.2 O.sub.4. U.S. Pat. No. 5,196,279 teaches the use of Li.sub.1+x Mn.sub.2 O.sub.4 as a cathode material for lithium ion batteries.
One of the attractive features of the present lithium ion electrochemistries is the high voltage provided by a single cell. Many electronic circuits require voltages of 3 V or 6 V for their operation. A battery to power such circuits could consist of series connected strings of 3 or 5 nickel-cadmium cells (1.2 V per cell) respectively or by 1 or 2 three volt lithium ion cells respectively. Use of the latter greatly simplifies the battery assembly and packaging required for such applications with a corresponding possible significant reduction in overall battery cost. In general, increasing the voltage of a single cell leads to a requirement for fewer series connected cells in a battery application, which is obviously desirable. Additionally, higher voltage is generally desirable for increased energy density, since the stored energy in a battery is given by the product of the average battery voltage times the capacity.
The voltage of a lithium ion battery is determined by the difference between the chemical potential of the inserted lithium in each of the two electrodes. To maximize the battery voltage, it is thus desirable to maximize this difference in chemical potential. For example, in the battery based on Li.sub.x Mn.sub.2 O.sub.4 /graphite (Li.sub.v C.sub.6) electrochemistry described by J. M. Tarascon et al, Electrochimica Acta 38, 1221 (1993), the chemical potentials are approximately -4.1 ev and -0.1 ev versus metallic lithium for Li.sub.x Mn.sub.2 O.sub.4 and Li.sub.y C.sub.6 respectively. These chemical potentials reflect the binding energies of lithium within the respective insertion hosts measured with respect to lithium in lithium metal. The lithium in Li.sub.x Mn.sub.2 O.sub.4 is much more tightly bound than is the lithium in Li.sub.y C.sub.6. The resulting voltage across the terminals of the battery is thus about 4.1-0.1 or 4 volts, with the Li.sub.x Mn.sub.2 O.sub.4 electrode as positive.
Practical application of such high voltage lithium ion electrochemistries is made difficult as a result of stability problems with other battery components at these voltages. Both the electrolyte and the hardware are subject to oxidation at the cathode, thus placing limitations on the choice of both. Aluminum appears to be a practical material for cathode hardware in most electrochemical systems. The problem of selecting an electrolyte that combines oxidation resistance along with other requirements (such as safety) remains an issue. Guyomard et al, U.S. Pat. No. 5,192,629 show how the judicious selection of the proper electrolyte can minimize oxidation for a given system. The system described therein included a carbon anode and a LiMn.sub.2 O.sub.4 cathode with an electrolyte based on dimethyl carbonate and ethylene carbonate solvents and preferably LiPF.sub.6 salt.
Lithium transition metal oxides with an atomic structure known as inverse spinel have been described in the literature as early as 1961 (eg. Bernier et al, Comptes Rendus, 253, 1578), however it appears that these materials have never been considered for use as electrodes in lithium batteries. This may be a result of the inverse spinel structure differing significantly from that of the more familiar compounds LiNiO.sub.2, LiCoO.sub.2 and LiMn.sub.2 O.sub.4 and appearing to be unsuited for use as battery electrodes.